The production of oil, gas, water, or any combination of these three are produced from wells penetrating the earth's subsurface strata. The wells are most often completed with casing (and liners) cemented through to the productive strata in the subsurface. Wells are also occasionally completed with uncemented liners. In either case, perforations or slots must be made through the casing and cement (if present) to provide a flow path for fluids from the productive strata into the casing. Fluids which have reached the inside of the casing via the perforations or slots may then be produced to the surface. However, the openings which, for example, may be slots in the liner preformed on the surface and/or perforations opened in the casing and formation, will often become plugged.
If a perforation tunnel in the casing, cement sheath, or formation becomes obstructed, then fluid flow will cease or will be impaired. This problem is especially serious in areas where hard, insoluble scales plug perforations. In any event, removal and replacement of the casing or liner is costly and is only a temporary solution since the casing or liner, as well as the adjacent formation, will eventually again become plugged.
Sections of recovered plugged casing and liner have been analyzed to determine the identity of the plugging material. Results have shown that the plugging material is mostly inorganic. Generally, it appears to be fine sand grains cemented together with oxides, sulfides and carbonates. Some asphaltenes and waxes are also present. Where water is produced, scale also seems to be present and presents a very tough plugging material. Examples of scale include barium sulfate, strontium sulfate, and silicates.
Many methods for cleaning openings in well casing or liners have been heretofore suggested. There have been three general methods employed which may be classified as 1) mechanical, 2) chemical, and 3) hydraulic.
Mechanical methods can be thought of as using physical force to scrape an obstruction from the perforation tunnel. There are no prior art mechanical means to effectively clean perforations. Mechanical methods at this time are limited to cleaning inside the casing, which does not address the perforation itself. The only mechanical alternative to deal with obstructed perforations is to drill and complete a new wellbore, which is usually economically unattractive.
Mechanical methods of cleaning the openings in casing or liners include the use of scratchers and brushes to cut, scrape or gouge the plugging material from the perforations. There are many disadvantages of these approaches. For example, the knives or wires in the brushes must be very thin to enter the slotted perforations which generally measures only 0.040 to 0.100 inches wide and, therefore, the knives and wires are structurally weak. Thus, an insufficient amount of energy is generally applied to really unclog the perforations. Furthermore, the cleaning tool must be indexed so that the knives or wires actually hit a perforation. Since only 3% of the casing or liner surface area is generally perforated, the chances are not favorable for contacting a perforation.
Chemical methods usually consist of using some chemical agent to dissolve or dislodge obstructions in the perforation tunnel. Common chemicals used to remove obstruction are acids, aromatic solvents, alcohols, and surfactants. These chemicals have been found to be very effective at removing a wide variety of obstructions in and around perforation tunnels. The chemical methods require that the obstruction be chemically reactive with the chemicals placed in the perforation tunnels. However, there are a number of substances which are essentially non-reactive and inert for all practical purposes. Some common examples of these relatively inert obstructions are barium sulfate, strontium sulfate, and silicates. These substances are frequently deposited as scales. The deposition of these scales in and around perforation tunnels can obstruct or impede fluid flow.
Chemical solvents have been developed which purport to dissolve these non-reactive substances. These solvents have been evaluated in the laboratory and in field trials, and have been found to be very ineffective. The chemical solvents were found to dissolve such a small amount of these non-reactive substances that they are economically unattractive.
The combinations of plugging materials often inhibits the reaction of the chemicals. For example, an oil film will prevent an acid from dissolving a scale deposit and a scale deposit will prevent a solvent from being effective in dissolving heavy hydrocarbons. The chemicals cannot always be selectively placed where they are needed due to varying permeabilities encountered in a well bore and/or they dissolve the material in a few perforations and then the chemicals are lost into the formation where they can no longer be effective in cleaning the perforations.
Hydraulic methods include pumping a fluid between two or more opposed washer cups until the pressure builds up sufficiently to hydraulically dislodge the plugging material. Explosives such as primer cord (string shooting) have been used to form a high energy pressure shock wave to hydraulically or pneumatically blow the plugging material from the perforations. The disadvantages of these two methods are that the energy is applied non-directionally to the casing or liner and it always takes the path of least resistance. The use of these methods generally results in opening only one or two perforations out of a perforation row containing from 16 to 32 perforations.
Jetted streams of liquid have also been heretofore used to clean openings. The use of jets was first introduced in 1938 to directionally deliver acid to dissolve carbonate deposits. Relatively low velocities were used to deliver the jets. However, this delivery method did improve the results of acidizing. In about 1958 the development of tungsten carbide jets permitted including abrasive material in a liquid which improved the ability of a fluid jet to do useful work. The major use of abrasive jetting has been to cut notches in formations and to cut and perforate casing to assist in the initiation of hydraulically fracturing a formation. The abrasive jetting method requires a large diameter jet orifice. This large opening required a large hydraulic power source in order to do effective work. The use of abrasives in the jet stream permitted effective work to be done with available hydraulic pumping equipment normally used for cementing oil wells. However, the inclusion of abrasive material in a jet stream was found to be an ineffective perforation cleaning method for use with liners in that it enlarged the perforation which destroyed the perforation's sand screening capability. A jet that uses abrasives also is likely to cause casing damage.
Another method for directionally applying a high pressure jet to a well liner to clean openings in the liner which are plugged with foreign matter has been suggested. High pressure liquid jets having a velocity in excess of 700 feet per second are jetted at the liner from jet orifices having a standoff distance less than 10 times the diameter of the orifice to remove plugging material from the liner openings. An apparatus for concurrently circulating foam is provided in combination with the apparatus used to deliver the high pressure, high velocity jets, due to the relatively low circulation rate.
Relatively small diameter, threadably attached orifices which produce jets of 1/16th of an inch or less were thought to be advantageous in this method. A preferred orifice diameter for use in accordance with the method was 1/32nd of an inch. The use of small diameter threadably attached jets was thought to be very advantageous in that liquid volume requirements are lowered, thus lowering horsepower requirements and reducing the possibility of formation damage in low pressure formations caused by liquid in the well overpowering the formation. For example, see U.S. Pat. Nos. 3,850,241; 4,088,191; 3,720,264; 3,811,499; and 3,829,134; each of which issued to S. O. Hutchison. Whereas Hutchison's invention was a substantial improvement over the prior art at the time regarding cleaning perforations in a casing or liner, his method did not provide a means to clean out the perforations in the geologic formation itself, adjacent to the perforations in the casing or liner, or to adequately remove insoluble scale. The cleaning radius of Hutchison's tool is limited by the small nozzles used (1/32nd of an inch). The retained energy of jets is a function of the number of nozzle diameters from the point of origin. Using water (without chemical additives) the effective cleaning range of a nozzle is typically taken as 10 nozzle diameters due to energy decay. This results in effective cleaning radius of up to 5/16ths of an inch for a 1/32nd of an inch nozzle.
The addition of high molecular weight polymers results in enhanced jet performance. The effective cleaning range of a nozzle can be extended out to 100 nozzle diameters. The Hutchison tool with the use of polymer would then have a cleaning radius of up to 31/8 inches. Typical perforations, usually extend from 3/16 of an inch out to 15 inches radially from the nozzle. Thus, the Hutchison tool can only clean a small fraction of the perforation tunnel, and fluid flow remains greatly impaired.
Using larger nozzles, in the range of 1/16th to 1/4 inch, larger cleaning radii can be obtained. For the case of 1/8th inch nozzles, the effective cleaning radius can be increased four fold over Hutchison's tool to 121/2 inches. This larger cleaning radius results in more of the perforation being cleaned, and hence improved fluid flow.
Hutchison, as well as the other prior art, actually taught away from using larger nozzles in an effort to clean the perforations in casing. Hutchison maintained that the use of relatively smaller diameter jet orifices of less than 1/8 inch has the advantage of reducing to a minimum the amount of liquid being injected into the well, as well as reducing horsepower requirements. Also, Hutchison incorporated threadably attached, specially designed jet nozzles and made no mention of nozzles being attachable by 0-rings.
A further attempt to improve the existing methods was made by C W. Zublin. Zublin, a licensee of the Hutchison patents, received U.S. Pat. Nos. 31,495; 4,441,557; 4,442,899; and 4,518,041. U.S. Pat. No. 31,495 added a centralizer to help center the jet nozzles and provide a means to pan out of tight places in the tubing. This device is rotated by a power swivel at the surface. Zublin, however, maintained that larger nozzles are disadvantageous in that they cause a pressure drop, and recommended that the jet orifices be only 0.03 (1/32) inch in diameter. Zublin also only taught the use of threadably mounted nozzles.
U.S. Pat. No. 4,441,557 claims nozzles spaced so as to direct cleaning fluid onto the pipe in a certain pattern. The device is rotated at a constant speed by the power swivel at the surface. Again, 0.03 (1/32)-inch threadably mounted nozzles were used, as larger nozzles were said to cause a pressure drop.
U.S. Pat. No. 4,442,899 claims a method and a system for a non-rotating device utilizing threadably mounted 0.0325 (1/32-inch) nozzles and alternating pressure to create an oscillating twisting force according to a certain formula, for use with coiled tubing.
U.S. Pat. No. 4,518,041 claims a method and a system utilizing a device that is not rotated by the tubing at the surface. The device has threadably mounted 0.0325 (1/32-inch) nozzles which, like the device in U.S. Pat. No. 4,442,899 direct the flow of the cleaning fluid in such a manner as to tend to twist the tubing.
A further attempt to improve the well cleaning process was made by Wm. H. McCormick, who received U.S. Pat. No. 4,625,799. U.S. Pat. No. 4,625,799 claims an apparatus for pressurized cleaning of flow conductors. The device utilizes a control slot which assists in indexingly rotating the nozzle section. Neither nozzle size nor means of nozzle attached are discussed.
The above methods and devices are all limited in the effective cleaning distance of the jets, to a distance of up to 10 times the diameter of the jet orifice. Also, none of the prior art teaches a method of how to remove insoluble scale, such as barium sulfate, strontium sulfate, or silicate. This limitation prevents actual cleaning of the perforation tunnels in the adjacent production geologic formation, which often become plugged and therefore inhibit oil or gas production. There is, therefore, still a need for a method of cleaning openings both in a well casing or liner and in the adjacent geologic formation which is a practical and relatively easy operation to perform. Further, there is need for a method of cleaning openings in such casings, liners, and geologic formations which does not destroy or alter the openings or damage the casing or liner.
The above methods and devices are also limited in that the nozzles must be specially designed to be threadably attached to the cleaning tool. Constructing the individual nozzles is relatively expensive. There is therefore still a need for a method of attaching readily available, relatively inexpensive nozzles to the cleaning tool.